146 
THE TERTIARY VOLCANOES 
BOOK VIII 
miles, running occasionally for a short distance at a right angle to its general 
direction (see Eig. 257). 
Among these solitary dykes also, though the persistence of their trend 
is so predominant, there occur instances where the general direction under- 
goes great change. Some of the most remarkable cases of this kind 
have been mapped by Mr. B. 1ST. Peach and Mr. E. L. Jack, in the course 
of the Geological Survey of Perthshire. Several important dykes strike 
across the Old Ked Sandstone plain for many miles in a direction slightly 
south of west. But when they approach the rocks of the Highland border 
in Glen Artney, they bend round to south-west, and continue their course 
along that new line. 
Many years ago I called attention to the dominant trend of the dykes 
from north-west to south-east. 1 Subsequent research has shown this to be on 
the whole the prevalent direction throughout the whole region of dykes. But 
the detailed mapping, carried on by my colleagues and myself in the Geological 
Survey, has brought to light some curious and interesting variations from 
the normal trend. In the districts where dykes of the gregarious type 
abound there is sometimes no one prevalent direction, but the dykes strike 
to almost all points of the compass. Of the Arran dykes, so carefully 
catalogued by Necker, only about a third have a general north-westerly 
course. But in Eastern Argyleshire the abundant dykes mapped by Mr. 
Clough trend almost without exception towards N.N.W. In the North of 
Ireland, Berger found the direction of thirty-one dykes to vary from 17° to 
71° W. of N., giving a mean of N. 36° W. 2 In Islay, Jura, Eigg, Mull, 
and Skye the mean of several hundred observations has given me similar 
results. Among the Inner Hebrides, however, though the general north- 
westerly trend is characteristic, many of the later dykes show marked 
departures from it. Thus in Strath, Skye, some of the youngest follow a 
nearly north and south direction (Fig. 253). In the Blath Bhein hill- 
range, Mr. Harker has found that the latest dykes cut the gabbro at right 
angles to the prevalent trend and are further distinguished by their low 
hade. 
It appears, therefore, that though there is sometimes extraordinary local 
diversity in the direction of the dykes in those districts where they present 
the gregarious type, the general north-westerly trend can usually still be 
recognized. But when we turn to the long massive solitary dykes, we soon 
perceive a remarkable change in their direction as we follow them northward 
into Scotland. I formerly pointed out how the general north-westerly 
trend becomes east and west in the Lothians, with a tendency to veer a little 
to the south of west and north of east. 3 This departure from the normal 
direction is now seen to be part of a remarkable radial arrangement of the 
dykes. Beginning at the southern margin of the dyke-region, we have the 
notable example of the Cleveland dyke, which in its course from Cleveland 
to Carlisle runs nearly W. 15° N. The Eskdale dyke has an average trend 
1 Trans. Roy. Hoc. Edin. xxii. (1861), p. 650. 2 Trans. Geol. Soc. iii. p. 225. 
s Trans. Roy. Soc. Edin. xxii. p. 651. 
